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The dead time is there to ensure that the high and low side fets/bjt's are never on at the same time , thus shorting the supply. It depends on how quickly you can turn on and off the fets that will dictate the dead time. Some driver ic's(ie ir2104) have a fixed dead time (500ns for the 2104 if I remember correctly).
If your driver cct can't turn the fets "off" quickly , you may need a longer dead time to ensure the top fet is off before turning "on" the bottom one. If you get it wrong , it's fairly easy to spot :0)
There are a lot of variables(eg . gate charge , driver current capability).
Build the circuit and test it!! Just put in some current limiting.. :0)
can u explain what do you mean by There are a lot of variables(eg . gate charge , driver current capability).
Build the circuit and test it!! Just put in some current limiting..
I'm not sure which part you want me to explain.
The speed at which you can turn a fet on/off is infuenced by the current capability of the driver. The more current it can source/sink , the faster you'll turn the FET ON/OFF.
The gate charge of the FET also plays a part. The larger the gate charge of the fet , the longer it will take to turn ON/OFF.The value of the Gate resistor that you use will also determine the speed at which you can turn on/off the fet.
Build a test circuit , get out your scope and probe around and get a feel for what's going on.It's easy enough to see everything you need to. Just remember to put in some current limiting. If you do end up with a high side and low side fets on at the same time , you want something to limit the current. In my applications which are mostly off-line supplies I use a 60W 220V lightbulb in series with the mains supply , to limit the current. If you have a large bulk cap after the bridge it can have enough energy to blow a fet if things go wrong , but it won't be lifting tracks and destroying the board.
The basic point of dead time is to prevent cross conduction of the top and bottom power mosfets, or you get BOOM and SMOKE.
Sometimes the controller that drives the H-Bridge adds a fixed amount of dead time to the logic signals driving the top and bottom mosfets.
You must realize that applying a logic signal to the input of an H-Bridge driver does not appear instantly on the output. There is propagation delay through the driver. There is the time it takes to turn off the gate of the power mosfet (it acts like a capacitor). There is propagation delay change over temperature, so a worst case must be considered. There is differences between propagation delay paths to the top mosfet and bottom mosfet, so worst case must be considered. There is differences between the rising propagation delay and falling propagation delay, so worst case must be considered. There is even the difference between the voltage applied to the gate terminal of a power mosfet and what is actually seen on the physical gate of the mosfet (gate resistance and inductance are effective at high frequencies)
Not to discourage you, just letting you know there are things to consider. Longer dead times are always safer, but it can affect efficiency (if that is important to you)
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